It is known that women are less able to tolerate upright posture than men. While it is not known precisely why this is true, we think that it may be because women do not constrict their blood vessels as much as men when they stand up, and therefore their blood pressure is not maintained as well. It is the hypothesis of this investigation that estrogen, a female hormone, may contribute to these differences. While estrogen generally has a protective effect on the heart and blood vessels, it may contribute to this reduced tolerance of upright posture. We have noticed that women astronauts have more trouble with orthostatic hypotension than men astronauts and we are tying to determine the reasons for this.